textproduct: Albany

WHAT HAS CHANGED

Confidence in snowfall amounts nearing or exceeding 7 inches increased enough in southern Vermont and the southern Adirondacks that we issued a Winter Storm Warning starting tomorrow morning and continuing through Saturday.

The rest of eastern NY and western New England will likely experience a wintry mix of wet snow, sleet, and pockets of freezing rain tomorrow through Friday night. We therefore issued Winter Weather Advisories for most areas, except in valley areas from the immediate Capital District into the mid-Hudson Valley where freezing rain is less likely. If confidence in freezing rain increases, we may need to expand winter weather advisories.

KEY MESSAGES

1) There is a medium to high chance for wintry mixed precipitation including snow, sleet, and freezing rain to impact much of eastern New York and western New England Friday into Saturday. This will likely lead to slippery travel for the Friday evening commute.

2) Continuing to monitor an off-shore coastal low Sunday night into Monday. Probabilistic guidance shows a 10 to 25% chance for 3 inches or more of snowfall amounts mainly south of I-90.

DISCUSSION

KEY MESSAGE 1...

While Canadian high pressure will dominate the Northeast through early tomorrow morning keeping us dry, we continue to monitor a boundary positioned to our south that will lift northward as a warm front, resulting in overrunning precipitation tomorrow morning. As mentioned in our previous discussion, initial dry air from the high should delay the start time of the wintry precipitation until closer to or shortly after 15 UTC (mainly after the A.M commute) when the ridge axis finally lifts to our north and east and warm air and moisture advection kicks in. While initial temperatures are in the low to mid 30s, diabatic cooling from wet-bulbing as precipitation falls the dry air and moistens the column should support an initial period of snow that lasts through early afternoon. Snow should then transition over to a wintry mix by 18 to 21 UTC as southwesterly flow in the low and mid-level jet strengthens the warm nose enough to result in snow mixing with/changing to sleet and even freezing rain in the northern/eastern Catskills, Mohawk Valley, Taconics and western New England where sfc temperatures will be closer to freezing. On the other hand, the Hudson Valley will likely see initial wet snow transition to sleet and then mainly plain rain tomorrow afternoon as south to southeasterly flow funnels warm air up the valley allowing sfc temperatures to warm into the mid-30s. For these reason, we held off on a winter weather advisories in the Hudson Valley from the immediate Capital District into the mid-Hudson Valley. However, if temperatures trend cooler, we may need to expand advisories. With wintry mix ongoing during the Friday evening commute, expect slippery travel conditions and allow for extra time to reach your destination.

By early Friday evening, the mainly area of warm air/moisture advection lifts to our north and the pseudo-dry slot noses into the region which should decrease precipitation intensity but maintain the wintry mix of freezing rains/sleet/snow for a short period. Then, the closed parent cyclone arrives from the west by 00-03 UTC resulting in increased cyclonic vorticity advection and enhancing precipitation intensity once again. This second period of steadier precipitation Friday evening looks to fall as mainly snow (becoming dry/light in nature) as the main warm nose will have exited to our east and westerly winds advect a cooler air mass eastward. The latest HREF guidance even shows 20 to 40% chance for snowfall amounts to exceed 1" through Midnight mainly near and north of I-90. While the strongest forcing for ascent exits to our east after 06 UTC allowing snowfall rates to decrease, we still expect lingering snow showers overnight due to continued cold air advection and westerly flow as the parent low becomes sheared out overhead.

All in all, expecting 1 to 4 inches of snow for most areas in Winter Weather Advisories with 4 to 8 inches in Winter Storm Warnings. The initial period of snow daytime Friday will likely be wet/heavier in nature especially where sleet falls before snow turns lighter/drier Friday night. In addition, expecting a glaze up to a tenth of an inch of flat ice from freezing rain with the highest amounts ranging from a tenth to two tenths of an inch in western New England, the northern/eastern Catskills, the Taconics, and Mohawk Valley where sfc temperatures will likely stay cooler.

Daytime Saturday will continue to see scattered snow showers as westerly flow continues with upslope snow showers in the southern Adirondacks, the Taconics and southern Greens where there is a 15 to 30% chance for 2 inches or more of additional snow.

KEY MESSAGE 2... Latest EPS and GEPS ensemble guidance continues to show a coastal storm for Sunday afternoon into Sunday night tracking well to our south and east. The GEFS shows most of the individual members and ensemble mean slight further north and west. The ensuing coastal storm develops in response to a parent shortwave trough from the Midwest phasing with southern stream energy and quickly amplifies as it near the Appalachians in the mid-Atlantic. Very strong height falls off the mid- Atlantic coast leads to a coastal low that undergoes rapid cyclogenesis as the trough becomes negatively tilted. However, given the displacement of the strongest forcing to our south including the left exit and right entrance region of a dual jet structure, it makes sense that the ensembles and ensemble mean keep most tracks south of the 40N/70W benchmark. The latest probabilistic guidance still shows 10 to 25% chance for at least 3 inches of snow focused mainly south of I-90 from 7AM Sun to 7AM Mon but this snowfall may actually stem more from the parent trough than the actual coastal storm. With all that said, we will continue to closely monitor this coastal storm but the latest probabilistic WSSI output from WPC only shows 10% chance for minor impacts focused in Ulster/Dutchess/Litchfield County between 1PM Sun and 1PM Mon.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Through 00z Saturday...Mainly VFR flying conditions as of 6:15 PM EST, although cigs are MVFR at PSF. ALB/GFL/POU should see mainly BKN to OVC mid-level clouds 3500-4000 ft through late tonight, then trending to MVFR cigs prior to daybreak. At PSF, mix of MVFR/VFR cigs expected through tonight, becoming mainly MVFR prior to daybreak as well.

Tomorrow morning, precipitation spreads northeastwards across the region. Precipitation starts as mainly snow everywhere, with IFR or lower vsbys and MVFR to IFR cigs. At POU, precipitation changes fairly quickly to a brief wintry mix and then rain, with mainly MVFR conditions through the afternoon. For ALB/PSF, a few to several hours of snow are expected before the changeover to a wintry mix of snow, sleet, and freezing rain. Some plain rain may mix in at ALB towards the end of the TAF period. Low confidence in exact timing of the change in precip types, with further refinement likely in subsequent TAFs. That said, mainly IFR to LIFR vsbys with the snow at both terminals, trending to IFR vsbys and cigs at ALB and IFR vsbys / MVFR to IFR cigs at PSF once snow changes to a mix. At GFL, mainly snow expected, although some sleet could mix in towards the end of the TAF period. Here, mainly IFR to LIFR vsbys and cigs expected late morning through 00z Saturday.

Winds will be light and variable tonight, increasing to around 5 kt from the E/SE tomorrow mid-morning through the end of the TAF period, except at PSF where easterly winds of around 10 kt with gusts to 20 kt are expected tomorrow afternoon.

Outlook...

Friday Night: High Operational Impact. Definite SHRA...SHSN. Saturday: Moderate Operational Impact. Chance of SHRA...SHSN. Saturday Night: Moderate Operational Impact. Chance of RA...SN. Sunday: Moderate Operational Impact. Chance of SN. Sunday Night: Moderate Operational Impact. Chance of SN. Monday: Moderate Operational Impact. Chance of SN. Monday Night: No Operational Impact. NO SIG WX. Tuesday: No Operational Impact. NO SIG WX.

ALY WATCHES/WARNINGS/ADVISORIES

CT...Winter Weather Advisory from 10 AM Friday to 1 AM EST Saturday for CTZ001-013. NY...Winter Weather Advisory from 10 AM Friday to 1 AM EST Saturday for NYZ041-043-050-054-061-066-084. Winter Weather Advisory from 7 AM Friday to 1 AM EST Saturday for NYZ032-038>040-047-048-051-058-063-082. Winter Storm Warning from 10 AM Friday to 1 PM EST Saturday for NYZ033-042-083. MA...Winter Weather Advisory from 10 AM Friday to 1 AM EST Saturday for MAZ001-025. VT...Winter Storm Warning from 10 AM Friday to 1 PM EST Saturday for VTZ013>015.